After you get the ragnarok in the 3rd disc look around the sea until you find a structure. Land and go in the building. You have to follow a certain pattern or lots of enemies will attack you, even if you have enc. none on. Try to get to the glowing area in the middle. When you do, you will have to fight the bahamut gf.

After you defeat him you'll not only get him, but there will now be a hole in the floor. Now you will have to go down a bunch of floors and then some stairs. When you get down you will have to fight ultimate weapon. While fighting him draw the spell named eden and after the battle you will have the eden gf !